This is an outdated and unique Japanese culture called "Inkan". It is common practice to stamp this seal on documents and other items as proof of identification and approval. As of 2021, there are finally more and more cases in Japan where "Inkan" are no longer needed for various contracts, so these seals will disappear with the times. So, these are valuable NFTs. Written on each "Inkan" is a famous Japanese surname. There is only one NFT per surname. Are there any Japanese surnames that you know?